The present invention relates in general to a time base circuit for producing a sampling strobe signal in a waveform sampling system and in particular to a time base circuit which provides for an accurately adjustable delay time between a triggering event and the strobe signal.
Waveform sampling systems typically utilize a strobed sampling gate to sample an input waveform and it is desirable that a strobe signal controlling gate operation be generated a predetermined delay time following a triggering event, such as a zero crossing in the waveform being sampled. For example an averaging sampling system samples a periodic waveform at a similar point along several repetitive sections of the waveform in order to determine an average value of the waveform at that point. To ensure that the waveform is sampled at the same point along each repetitive section of the waveform, each successive strobe signal should be generated at the same relative time with respect to a repetitive triggering event in each section of the waveform. As another example, an "equivalent time" sampling system samples several points at regular intervals along successive sections of a repetitive waveform by progressively increasing the strobe signal delay by regular, predetermined amounts after each repetitive triggering event in the waveform. This enables the sampling system to obtain samples representing several point along one cycle of the waveform even though the frequency of the waveform is relatively high compared to the sampling rate of the system.
The accuracy of averaging and equivalent time sampling depends on the accuracy with which the delay between the triggering event and the sampling strobe signal can be controlled. In sampling system time base circuits of the prior art, the input waveform is applied to a trigger generator which produces a trigger signal on detection of the triggering event in the waveform. The trigger produced by the trigger recognizer is then delayed by a programmable delay circuit before being applied to a strobe signal generator and the strobe signal generator produces the strobe signal in response to the strobe drive signal. The time delay between the trigger signal and the strobe signal is controlled by programming the delay circuit, taking into account any inherent time delay in the operation of the trigger recognizer circuit. However the inherent operating time delay associated with the trigger generator and the programmable time delay of the delay circuit are influenced by ambient temperature fluctuations and therefore the timing of successive strobe signals with respect to successive triggering events can vary with time. What is needed and would be useful is a method and apparatus for continuously and rapidly recalibrating the sampling delay time.